Friday, October 7, 2011

Nebulae, galaxies and star clusters – they may be distant, but they are also huge. Many of them are so big, in fact, that they appear larger than a full moon in the night sky. So why can you not see them? They are difficult to see not because of their size, but because they are faint. This is why the purpose of telescopes is not just to magnify the sky but to gather a lot more light than the naked eye can.

Jupiter and Saturn are among the smallest objects in the sky. Most of the pictures of nebulae, galaxies and star clusters that you have seen range in size from the size of Jupiter, to larger than a full moon. Examples of well known pictures that fall into this range are: The Orion Nebula, the Horsehead Nebula, the Andromeda Galaxy, the Whirlpool Galaxy, the Crab Nebula, the Eagle Nebula, and even the Hubble Deep Field images – along with many, many others.

A short discussion of units is important. Apparent size, which is the size that your eye or a camera perceives something to be, is measured in degrees. For example, the Moon is about 0.5° degrees across. This means that if you drew a line from the left side of the Moon all the way to your eye, and another line from the right side of the Moon all the way to your eye, the angle between the lines is 0.5° degrees. Degrees are divided into 60 arcminutes. The Moon is, therefore, about 0.5 * 60 = 30 arcminutes across.

Below I have a collection of to-scale images of celestial objects. All of these are scaled relative to how big they actually look in the night sky. If you stand back 22 feet from your computer screen, these images will actually look the same size as they are in the sky. This is true not only of the Moon, but also of ALL the pictures in this post.

The Basics - To Scale

The most familiar objects in the sky. Use this image as a reference for the other images. All of the images in this post are to scale.

The Great Nebulae - To Scale

These are probably the two most well known nebulae. Both are larger than a full moon. Notice the full moon partially visible in the lower right corner. This is for convenient size comparison.

The Great Nebulae - To Scale, Continued

Also very popular, these nebulae rival the full moon in size. The Eagle Nebula is home to the famous Hubble picture "Pillars of Creation". As a side note: the colors in these and most other pictures from space are artificial. Nearly all objects in space look white in real life. The artificial coloration is usually based on spectrum data that the human eye cannot perceive normally.

The Great Andromeda Galaxy - To Scale

The Andromeda Galaxy is huge - it is much larger than the full moon in the night sky. Even better, this galaxy is actually bright enough to see with the naked eye on very dark nights. That's right - you can just look up into the sky and see a whole galaxy. Astronomers believe that the Andromeda galaxy is heading toward the Milky Way, and may one day collide with it.

Famous Galaxies - To Scale

The Triangulum Galaxy is larger than a full moon. Like the Andromeda Galaxy, this too is visible to the naked eye. The Sombrero and Whirlpool galaxies are just two of many similar galaxies that are scattered about the sky.

The Pleiades - To Scale

The Pleiades ('plee-uh-deez) is a prime example of an open star cluster. Open clusters are very beautiful, and are great for amateur astronomers. Clusters are still visible even when light pollution washes out nebulae and galaxies.

The Virgo Cluster - To Scale

The Virgo Cluster is a large cluster of galaxies near the Milky Way (near is a relative term). The Virgo Cluster is the center of the Local Supercluster, of which the Local Group (Milky Way, Andromeda and Triangulum) is an outlying member. The galaxies are about the same apparent size as the dark seas and oceans on the Moon.

Assorted Small Objects - To Scale

These are just a few of the many nebulae available to amateur astronomers. The Crab Nebula is an example of a supernova remnant, which is a nebula left behind after a supernova. Notice how small the planets are compared to the other celestial objects. The famous deep field pictures are shown here in outline. Look these up on Wikipedia if you are interested. You may wonder why the ultra deep picture is larger than the deep one. This is because 'deep' refers to sensitivity, not magnification. The Hubble Ultra Deep Field photograph is very sensitive, with a total exposure time of eleven days.

Human Visual Acuity - To Scale

This picture illustrates the limits of human visual acuity. A person with 20/20 vision can make out details down to 1 arcminute across. The pixelated picture of the Moon shows the level of detail visible with 20/20 vision. The sideways E is from a Snellen eye chart on the 20/20 vision row. In theory, a person with 20/20 vision could just barely determine the direction of the E if it was in the sky at that size. Stand back 22 feet from your computer screen to view these images in actual size, and see what you think.

If you would like to experience some of this for yourself, there are lots of ways. One fun way to start is to pick up a pair of binoculars and start looking. You can use the Google Sky app for your smartphone to help you find objects. It makes finding celestial objects so easy, it is sinful.

Or, just start scanning the sky with your binoculars. Your are usually bound to find something. This can be rewarding even if you live near a city. City lights are annoying, but usually they are not as bad as they seem at first. Just pick a clear, moonless night, find a suitable location, and start scanning for anything you find interesting. Even if the light pollution is so bad that nebulae and galaxies are washed out, you can still look for star clusters, planets, and the moon. The moon, especially a partial moon (i.e. not full), is very beautiful up close.

If you are really serious, there are lots of great telescopes in the 300-500 dollar range. Some of them are 'Go-To' style, where you just tell the computer what to look at and it points the scope for you.

By far the best way, though, is to find an astronomy club (or observatory) that has public viewing nights. In the Washington DC area, The NOrthern Virginia Astronomy Club (NOVAC) has public viewing nights every month. These are where a bunch of club members set up their telescopes in a field, and members of the public are invited to look through them free of charge.

I hope you have enjoyed this post. All of the original images are from the WikiMedia Foundation. The data I used to size the images correctly is from Google Sky. Most of the other information is from Wikipedia. All of this is made possible by NASA.